1. Field of the Invention
The present invention relates to a high-frequency module and more particularly to a high-frequency module such as a voltage-controlled oscillator including bare chip active elements or bare chip ICs including active elements mounted on a multilayer substrate.
2. Description of the Related Art
FIG. 4 is a schematic illustration showing the essential portion of a conventional voltage-controlled oscillator. The voltage-controlled oscillator 1 contains a multilayer substrate 2. A bare chip IC 3, a capacitor 4, another electronic component 5, and other elements, are mounted on the multilayer substrate 2. A transistor in the oscillator stage defines an active element in the IC 3. Moreover, a bare chip transistor itself in the oscillator stage may be mounted instead of the IC 3. Furthermore, the voltage-controlled oscillator 1 is arranged such that an electrode pattern 6 is disposed on the multilayer substrate 2 and the electronic components mounted on the multilayer substrate 2 are connected.
The voltage-controlled oscillator 1 is constructed so as to have a circuit as shown in FIG. 5, for example. The voltage-controlled oscillator 1 contains a capacitor C1 connected between the power-supply voltage Vc and the ground. Moreover, one end of a strip line SL1 is connected to the power-supply voltage Vc, and the other end of the strip line SL1 is connected to the cathode of a variable capacitance diode D1 and one end of a capacitor C2. The anode of the variable capacitance diode D1 is connected to the ground. Furthermore, the other end of the capacitor C2 is connected to one end of each of a strip line SL2, a capacitor C3, and another capacitor C4. The other ends of the strip line SL2 and the capacitor C3 are connected to the ground. Furthermore, the capacitor C4 is connected to a voltage divider circuit including resistors R1, R2, and R3.
This voltage divider circuit is connected between the power-supply voltage Vb and the ground. Moreover, the power-supply voltage Vb is connected to the collector of the npn transistor Tr1 through a strip line SL3. The emitter of this transistor Tr1 is connected to the collector of another npn transistor Tr2, and the emitter of the transistor Tr2 is connected to one end of a resistor R4. The other end of the transistor R4 is connected to the ground through a parallel circuit of a strip line SL4 and a capacitor C5.
The connection point of the resistors R1 and R2 is connected to the base of the transistor Tr1. Furthermore, the connection point of the resistors R2 and R3 and the capacitor C4 are connected to the base of the transistor Tr2. Moreover, one end of a capacitor C6 is connected to the base of the transistor Tr1, and the other end of the capacitor C6 and the base of the transistor Tr2 are connected to the emitter of the transistor Tr2 through a capacitor C7. Moreover, the emitter of the transistor Tr2 is connected to the ground through a capacitor C8.
Furthermore, the connection point of the power-supply voltage Vb and the strip line SL3 is grounded through a capacitor C9. Moreover, the connection point of the strip line SL3 and the transistor Tr1 is connected to an output terminal through a capacitor C10 and simultaneously grounded through a capacitor C11. Furthermore, the connection point of the emitter of the transistor Tr1 and the collector of the transistor Tr2 is grounded through a capacitor C12.
In the voltage-controlled oscillator 1 shown in FIG. 4, the capacitor 4 that is disposed on the multilayer substrate 2 is the capacitor C12 connected to the collector of the transistor Tr2 in the oscillator stage, and one end of the capacitor 4 is connected to the collector of the transistor Tr2 provided in the IC3 and the other end is grounded. In this voltage-controlled oscillator 1, the electronic components are, for example, flip-chip mounted, and the IC3 is connected to an electrode pattern 6 through bumps 7. As a matter of course, these electronic components may be connected to the electrode pattern 6 by wire bonding. Then, a sealing resin 8 is filled between the IC3 and the multilayer substrate 2 to secure a sufficient mounting strength.
However, in such a voltage-controlled oscillator, when the sealing resin filled between the IC and the multilayer substrate is in contact with other electronic components, particularly a capacitor connected to the collector of a transistor in the oscillator stage, since the high-frequency impedance of the electronic component changes, fluctuations of the oscillator output level and the high-frequency level and deterioration of the C/N ratio, etc., are caused, and accordingly stable characteristics cannot be obtained. Furthermore, since thermal expansion coefficients of the sealing resin and the multilayer substrate are different, there is a problem in that the reliability in a thermal shock test and the reliability of the mounted electronic parts are degraded.
In order to overcome the problems described above, preferred embodiments of the present invention provide a high-frequency module in which the influence of a sealing resin used to secure the mounting strength of active elements on a multilayer substrate is minimized and stable characteristics and a high reliability are achieved.
According to a preferred embodiment of the present invention, a high-frequency module includes a multilayer substrate, a bare-chip active element mounted on the multilayer substrate, a resin for sealing the active element on the multilayer substrate, and a grounding capacitance connected to the active element. In the high-frequency module, a strip line provided inside the multilayer substrate constitutes the grounding capacitance.
Furthermore, according to another preferred embodiment of the present invention, a high-frequency module includes a multilayer substrate, a bare-chip active element mounted on the multilayer substrate, a resin for sealing the active element on the multilayer substrate, and a grounding capacitance connected to the active element. In the high-frequency module, the grounding capacitance is preferably constructed by a strip line provided inside the multilayer substrate and a capacitor provided on the multilayer substrate.
Furthermore, according to another preferred embodiment of the present invention, a high-frequency module includes a multilayer substrate, a bare-chip active element mounted on the multilayer substrate, a resin for sealing the active element on the multilayer substrate, and a grounding capacitance connected to the active element. In the high-frequency module, the grounding capacitance is preferably constructed by a strip line provided inside the multilayer substrate and a microstrip line provided on the multilayer substrate.
In these high-frequency modules, the active element is preferably a transistor or an FET (field-effect transistor) in an oscillator stage and a voltage-controlled oscillator including the active element, and the collector of the transistor or the drain of the FET is connected to the grounding capacitance to construct the voltage-controlled oscillator.
When a grounding capacitance to be connected to an active element is constructed to include a strip line disposed in the multilayer substrate, the number of electronic components to be mounted on the multilayer substrate is significantly decreased and the possibility that the sealing resin for increasing the mounting strength of the active element may contact the other electronic components such as the grounding capacitance, or others, can be eliminated. Here, when the active element is connected to the other electronic components by an electrode disposed on the multilayer substrate, the influence of a stray capacitance generated between the active element and the electrode is increased by the sealing resin attached to the electrode for connection, but when the strip line is disposed inside the multilayer substrate, generation of stray capacitance between the active element and the strip line is prevented, and accordingly fluctuations in characteristics of the high-frequency module are minimized ed.
Furthermore, also when a grounding capacitance connected to an active element is constructed to include a strip line disposed inside the multilayer substrate and a capacitor mounted on the multilayer substrate or constructed by a strip line disposed inside the multilayer substrate and a microstrip line disposed on the multilayer substrate, the active element and the grounding capacitance can be connected by the strip line, and accordingly stray capacitance caused by the sealing resin is minimized.
Moreover, when a voltage-controlled oscillator is constructed as a high-frequency module, it is possible to construct the voltage-controlled oscillator such that the sealing resin does not contact a grounding capacitance connected to the collector of a transistor or the drain of an FET in the oscillator stage, and, as a result, the voltage-controlled oscillator having minimal fluctuations in the characteristics can be obtained.